The present invention relates to a display composition which undergoes reversible color development and color fading by oxidation-reduction reaction, to a coloring pigment, and to a recording material made with said display composition. More particularly, the present invention relates to a coloring pigment having good solvent resistance, to a new display composition suitable for electrolytic recording systems, and to an electrolytic recording paper.
A recent advance in the field of information recording is the development of the lactone-type functional dye which develops a color by the aid of an acid catalyst. This dye is based on the modification of the structure of phenolphthalein. It finds use as a component of pressure-sensitive copying paper and heat-sensitive recording paper which are now in practical use. For their production, it is dispersed, together with a color former, into a binder and the resulting composition is applied to a substrate such as paper and polymer film.
Pressure-sensitive copying paper, which is called non-carbon paper, develops a color by the following mechanism. It has microcapsules containing a leuco dye such as crystal violet lactone. As the microcapsules are broken by external pressure, the leuco dye is released. On contact with a solid acid, it forms a color through the ring opening of the lactone ring. Examples of the solid acid include a variety of electron acceptors having a phenolic hydroxyl group and also include clay minerals.
Heat-sensitive recording paper develops a color by the following mechanism. It contains a leuco dye (such as fluoran dye) and a color former (such as bisphenol A). They are brought into contact with each other in the molten state at a specific position which is locally heated by the thermal head according to the information for recording. The contact causes the color former to release protons which open the lactone ring, leading to the color development.
The above-mentioned lactone-type functional dye forms a color based on the principle that the intermolecular contact between a leuco dye (as an electron donor) and a color former or solid acid (as an electron acceptor) oxidizes the leuco dye at the acid centers of the color former, converting it into a type which takes on a color. For this color development to be recognized as letters or images, it is necessary that the leuco dye and color former are kept separate when no recording is made and they are brought into contact with each other by a proper means at the time of recording so that color development takes place locally at the part of contact.
Unfortunately, conventional pressure-sensitive copying paper and heat-sensitive recording paper still have a disadvantage that color development takes place at parts where color should not appear and the color fades away with the lapse of time.
In the case of pressure-sensitive copying paper, the separation between the leuco dye and the solid acid at the time of non-recording is accomplished by the microcapsulation of the leuco dye. Therefore, the production of conventional pressure-sensitive paper necessitates the process for microcapsulation of the leuco dye, which poses a problem associated with low productivity and high production cost. Moreover, microcapsules have another disadvantage that they easily cause unnecessary color development when the copying paper is pressed inadvertently and their recording is not stable completely.
The recording stability is also affected by the performance of the color former, or the intensity of the acidity of the color former. In the case where the color former has a sufficiently high color forming performance and hence the leuco dye takes on the chromophoric structure even without the strong chemical bond with the color former, the contact between the two compounds is likely to disappear, leading to discoloration. This is the reason why considerable discoloration takes place when pressures-sensitive copying paper or heat-sensitive recording paper is left in a hot humid place or brought into contact with an organic solvent which readily dissolves the dye.
Moreover, since conventional pressure-sensitive copying paper and heat-sensitive recording paper are not expected to be used repeatedly, it is very difficult for them to undergo color development and color fading reversibly.
It has been known that the reversible color development and color fading due to pH change is possible with composite clay in which a clay mineral is combined with a dye and a quaternary ammonium ion such that the quaternary ammonium ion is intercalated between the layers of the clay mineral. Devices have been made to improve the color stability by increasing the affinity of the intercalant (quaternary ammonium ion) for the clay mineral. There is disclosed a composite coloring pigment in Japanese Patent Publication No. 8462/1975. It has such a structure that a basic dye having the structure of quaternary ammonium salt is held in zeolite and/or montmorillonite by ion exchange with the exchangeable cation present between the crystal layers of the zeolite an/or montmorillonite. There is also disclosed in Japanese Patent Laid-open No. 35753/1982 a coloring material which undergoes color development and color fading due to pH change. It is formed by introducing a dialkylaminomethyl group into a phthalein indicator, cationizing the indicator (or combining the indictor with a quaternary ammonium ion), and finally causing the cationized indicator to be adsorbed by a clay mineral. Furthermore, there is disclosed in Japanese Patent Laid-open No. 90573/1988 a lipophilic composition of composite coloring pigment, which is formed by the steps of reacting an acid dye with a cation active agent of quaternary ammonium salt type to give a hydrophobic colored complex, and causing the complex to be adsorbed by a water-swelling clay mineral such as montmorillonite. In addition, there is disclosed in Japanese Patent Laid-open No. 256724/1987 an electrically conductive inorganic powder to be used as an antistatic agent, which is formed by causing a compound of quaternary ammonium salt type to be adsorbed by an inorganic powder such as mica and clay.
According to the above-mentioned prior art technology, the quaternary ammonium ion constitutes a part of the molecular structure of the intercalant, or the quaternary ammonium ion in conjunction with the intercalant form the complex. In either cases, the dye and quaternary ammonium ion behave as a whole. It should be pointed out, however, that the prior art technology involves no idea of reversible color development and color fading which take place through the ion exchange between the quaternary ammonium ion and the dye.